I'm a little late in replying to this post, but I finally had the time to check it out and listen to Joe. I can understand some positive aspects of this, but there is something I wanted to elaborate a bit more on the part of how many images are created with the various sizes and being stored on the server taking up space.

Nowadays, host packages offer you huge disk space, so it would take a while to fill it up with a ton of images and doubt for "most" users will see this as a big problem. However, there is something that no one has taken in account...Backing up your website creates a file. The more images that get generated from each upload, the bigger your backup file will be. This can create many issues for people, so this is something they will have to think about.

I recently wrote an article "Have you ever seen what is in your Uploads folder?" where it gives people an overview of the shear number of images that are created when Media Sizes are added (or kept intact). It was primarily focused around the amount of disk space these images take up, but also talked about the size of backup files, especially if one uses a lot of images on a daily or weekly basis. I generally prefer the media sizes are set to 0, and any image I upload, I create the size I need...including optimizing them.

On the positive side of Adaptive Images (not responsive images) coming in 4.4, I can see how it could benefit load times in mobile devices, although we still have to consider disk space and backup file sizes.

Here is something to also consider (unless I missed it somewhere), that some designs require an image to fill an area or container 100% in width, that this new adaptive function (not responsive) will/should load an image version that is closest to but not smaller than the container it needs to fill. If it is smaller, we will end up with an image that does not fill the required area as it should, therefore affecting the layout. There is also the case of page backgrounds; does this function affect that as well?